1. Field of the Invention
The present invention relates to a coating apparatus and a method of applying a coating solution on a web with a die, more especially a coating apparatus and a method for forming a multi-layer or mono-layer film by applying on a supporter (hereinafter a web), such as a polymer film, paper, metallic foil and the like, a coating solution of photosensitive emulsifier, magnetic material solution, solution for providing antireflectivity and glare shielding ability, solution for providing visual field enlarging effect, pigment solution for color filter, surface protective solution and the like.
2. Description Related to the Prior Art
A multi-layer film is produced by forming a coating when a coating solution is applied on a web by a coating apparatus with a die such as a slot die. Recently, it is required that a thickness of a wet coating on the web is less than 20 μm, in order that the coating film has such effects as is previously expected. The thickness and conditions of the wet coating on the web are strictly determined. Accordingly the coating solution must be applied with high accuracy.
However, when in applying the coating solution on the web, several outer conditions make the thickness of the wet coating on the web uneven, which causes stripes and non-uniformity in the coating. The stripes and the non-uniformity become more remarkable when the thickness of the wet coating becomes smaller. Accordingly, Japanese Patent Laid-Open Publication No. 9-511682 discloses a technology of sharpening of lips of a slot die. Concretely, each lip land or lip end has an arc-shaped form whose radius should not be more than 10 μm. However, a small error in producing processes has a large influence on forming the coating, in this case.
The radius of the lip land is usually determined as a width (herein after land width) of a flat portion which is formed in a feeding direction of the web and usually called a lip land. When the radius of the lip land is shorter than 30 μm, the lip end or the lip land are easily broken, which causes to generate stripes. Therefore it is hard to continuously carry out the coating. Further as shown in FIG. 19, a coating solution 216 discharged from a slit 206 of a die 200 covers lip lands 201a, 201b of down- and upstream lips 202, 203. When the coating solution 216 widens on the lip land 201a of the downstream lip 202, the coating on a web 212 has an inadequate shape, and therefore the film products have stripes on a coating surface. It is hard to adjust a position of wet line, an edge of the coating solution 216, on the downstream lip 203.
It is already known that there are several reasons for generating the unevenness, which are, for example, the change of feeding velocity of the web, the cyclic change of distance between a back-up roller for feeding the web and the lip land of the downstream lip. The variation of the distance between the buck-up roller and the lip land is caused by decentering in the back-up roller. The cause of the decentering of the back-up roller is that the core of the back-up roller does not protrude, or that a cross-section of the back-up roller does not have a strictly circular shape. Note that the unevenness in the amount of discharging the coating solution is caused by the pump feeding the coating solution in pulse movement.
There are other causes of the unevenness, for example, in variable positional relation between a vacuum chamber and the coating apparatus or between a vacuum chamber and the web while the coating apparatus and the web are confronted to the vacuum chamber. Note that the vacuum chamber is provided near to a bead formed by the coating solution exactly before applied on the web, so as to keep an adequate situation for preventing an unstable form of the bead. In this case, the variation of the positional relation prevents the uniform aspirate of the air from the vacuum chamber. Accordingly the bead often vibrates to cause the unevenness. In order to apply the coating solution on the web adequately, the above situations therefor must be improved. However, there are large problems about cost and techniques for the improvement. Note that an air pressure is set to a lower level in the vacuum chamber than usually.
The applicant proposes in the Japanese Patent Application No. 2001-368113a method to prevent the generation of the unevenness. In order to form the bead uniformly, a lip land of the upstream lip is bend at less than 100°, and a position of an upper meniscus of the bead is fixed at an upstream edge of a lip land of an upstream lip. Further, the change of the conditions of the outer in applying the coating solution has a smaller influence on the shape of the bead. Accordingly the unevenness in the coating is effectively prevented. However, as the situation of applying the coating solution becomes worse in the above method, the bead is often split, which causes the stripe in the coating.
The bead must be formed uniformly by setting the pressure decrease as small as possible. The formation of the coating on the web has a large influence on the quality of the film product. When in applying the coating solution on the web, the unstable situation of the edges in the bead causes the deformation of the coating. When the pressure decrease is made too efficiently, the wet coating on the web widens more than the expected one to cause the unevenness of the wet coating on the web. Especially, when the amount of the wet coating becomes smaller in the both sides, then the usable area for the film product becomes smaller. Furthermore, part of the bead, especially edges thereof, in which the amount of the coating solution decreases, brakes easily.
In Japanese Patent Publication No. 2001-170542, a regulation member is provided with the coating apparatus for regulating the width of the coating solution. The forward end of the regulation member is positioned slightly forwards of the lip land so as to form the bead uniformly. However, effects of the coating apparatus in this publication become smaller when in using the coating solutions having high viscosity or when in forming thick layers.
When the gap between the web and the lip in the downstream side (hereinafter downstream lip) of the die is smaller, the thinner coating is formed. However, the upstream lip has none of such conditions. Therefore, the gap between the upstream lip and the web may become larger. Considering these conditions of the down- and upstream lips, two conditions are to be satisfied. First, the gap between the upstream lip and the web becomes larger to prevent the pressure loss in the upstream side of the bead. Secondly, the gap between the downstream lip and the web is smaller to make the coating thinner. Accordingly, an overbite formation of the die is proposed.
The die of the overbite formation is used for forming the coating in high accuracy. In such die, the upstream lip has larger distance to the web than the downstream lip. In Japanese Patent Laid-Open Publication No. 9-511682, the position of the bead is fixed to a downstream edge of the upstream lip to form the bead uniformly. Further, the applicant proposed in the Japanese Patent Application No. 2002-014772 a coating apparatus including the die in the overbite formation to form the coating in high accuracy. Note that a difference between the gap from the web to the upstream lip and that from the web to the downstream lip is determined as an overbite length.
When the overbite length is too small, the effect of the overbite formation is not large. When the overbite length is too large, a slight variation of pressure decrease has influence on the formation of the bead, and the bead is not formed uniformly, and split furthermore. Further, when the difference between the gap from the downstream lip to the web and the gap from the upstream lip to the web is too small, the bead is hardly fixed to the upstream lip with small value of the decreased pressure. When the difference is too large, the bead is fixed too much even with the small value of the decreased pressure. Accordingly, the overbite length is to be adjusted to an adequate value, which is determined in accordance with the following conditions, such as the kinds and the viscosity of the coating solution, the coating velocity of the coating solution, the thickness of the wet coating, and the like. However, high cost and high techniques are required for forming the lip of the overbite formation.
There is a type of the die in which an upstream end block has the upstream lip and a downstream end block has the downstream lip. The upstream and downstream end blocks are attached to the upstream and downstream lip bodies, respectively. In this type of the die, when one of the up- and downstream end blocks is broken or disassembled, it is changed. However, it is hard to have the same overbite length thereby.
Further, there is a method for measuring the overbite length. In this method, the gap from the protruding lip to the web is measured in high accuracy, while the gap from the other retracted lip to the web is often measured with a large error, or cannot be measured. The gap from the back-up roller to the web or the lip may be measured with a gage or in a method proposed in the Japanese Patent 2002-047078 by the applicant of the present invention.